Modulation control system



July 27,1943.

MODULATION CONTRQL SYSTEM Filed July 23, 1940 i4 P@. I.

AUDIO AMPLIFIER 2 FIQZ. M F [2 ANODE Z Aumo 27 HODULATED -Z AMPLIFIER 2522 3/ 335 3 I3? 28 r I26 7 T 29 NODULATED TRANSMITTER 1 a 38 AUDIOAMPLIFIER Invent or George N. Br own,

His Attorney.

G. M. BROWN 2,325,366

PatentedJuly 21, 1943 George M. Brown, Sootia, N. Y., assignor to Genon]Electric Company, a corporation of New York Application July 2:, 1940,"Serial pa amas:

1 Claim. (c1. 179-1715) This invention relates to radio transmitters"and more particularly to meansi'or controlling the amount of amplitudemodulation of the wave radiated thereby.

In a common type of radio transmitter acarrent through a modulationtransformer upon which a signal is impressed. In such transmitters ithas been proposed to control the amount of modulation of the carrierwave by controlling the amplification of the audio amplifier inaccordance with a voltage developed in a diode rectifier connected inshunt to the modulation transformer and the direct current'su'pply forthe carrier wave amplifier. The diode is so poled as to benon-conductive in the presence of unidirectional potential in thecircuit. In order that the diode shall become conductive and produce acontrol potential for the audio amplifier only when the unidirectionalpotential for the carrier wave amplifier is reduced to somepredetermined voltage greater than zero, the diode has been maintainedat some voltage positive with tential developed thereby for the audioamplifier has been superimposed upon a positive unidirectionalpotential.

' It is an object of my invention-to provide such a modulation controlsystem in which the diode becomes conductive before the unidirectionalpotential for the carrier wave amplifier is reduced to zero, and inwhich the control potential de-' It is another object of my invention toprovide such an improved modulation control system. in

which the controlled amplifier circuit may be operated with one sideconnected to'ground, since the control potential therefor is notsuperimposed on a positive unidirectional potential.

' The features of my invention which I believe to be novel are set forthwith particularity in the appended claim. My invention itself, however,

both as to its organization and method of operation, together withfurther objects and advantages thereof may best be understood by refer--ence to. the following description taken in connection with theaccompanying drawing in which Fig. l-illustrates a radio transmitterembodying my invention;' Figs.- 2 and 3 represent certain modificationsthereof.

. Fig. 1 illustrates an anode modulated transrier wave amplifier issupplied with direct currespect to ground. As a result, the controlpomitter' l0 comprising a carrier wave amplifier discharge device I]supplied with unidirectional operating potential through a modulationtransformer l2 from a direct current source l3. A

carrier wave generator ll, also supplied with unidirectional operatingpotential from the source I3, transmits a carrier wave through a tunedcir- -cuit I! to be amplified through the discharge device II. Thecathode it of the device H is connected to ground and the J anode l1thereof .is

connected through a tuned circuit l8 and through the secondary of thetransformer to the positive terminal of the source- 18, whose negativeterminal is connected to ground. The carrier wave from the generatori 4,after passing through the tuned circuit, l5 and device I I, istransmitted from the tuned circuit l8 to a coil i9 coupled therewith,which impresses the carrier wave on an antenna 20 from which the waveisradiated. The discharge device H is shown conventionally as a triode andconventional neutralizing means including a condenser 2| is providedtherefor.

Audio signals 1mm microphone 22 are amplifled through a variable gainamplifier 23 in cascade wtih an audio amplifier 2 4. The amplifier 24transmits the amplified audiosignal from the microphone 22 through thetransformer I2 and thus superimposes the amplified audio signal upon theunidirectional potential from the source IS. The potential of the sourceI3 i thereby modulated in accordance with audio signal and the amplitudeof the carrier amplified through the 'device II i consequently modulated in accordance with the amplified audio signal.

The radio transmitter with its modulating systion. The modulationcontrol system comprises a pair of serially connected resistors 25 and26 connected in shunt to the source l3 and the secondary of .thetransformer l2. A condenser 21, sufilciently large to have a lowimpedance for low frequency audio signal currents, is connected inshuntto the resistor 25. A diode rectifier 28, serially connected-with aresistor 29, is connected directional potential, so that the operatingpotential of the device H is reduced to a predetermined voltage greaterthan zero, a control potentialv is developed across the resistor 29,negative with respect to ground. The relative intensities of theamplified audio signal and the unidirectional potential, at which thiscontrol potentialis produced, depend on the relative sizes of resistors25 and 26, as will be explained later.

This negative control potential is transmitted through a resistor 30 anda second resistor 3| to a control electrode 32 of the variable gainamplifier 23. As the negative potential of the control electrode 32 isincreased, the gain of the device 23 is reduced. The cathode 33 of thedevice 23 is connected to ground through a cathode resistor 34, bypassedfor audio frequency currents through a bypassing condenser 35. Spacecurrent flowing through the resistor 34 maintains the cathode 33 at aconstant positive potential, which maintains the control electrode 32 ata negative bias potential with respect to the cathode 33 to providemaximum gain through the device 23 when no voltage exists across theresistor 29.

It may be noted at this point that the cathode 33 of thedevice 23 issubstantially at ground potential, the voltage across the resistor 34being small. It is possible to have this amplifier clrcuit thus groundedbecause of the fact that the control potential developed across theresistor 29 is not superimposed upon a large unidirectional potential,which would make it necessary that circuits associated with the device23 be maintained at a substantial potential with respect to ground.

Since the control potential developed across the resistor 29 ispulsating in character, a low pass filter, including the resistor 39 andcondensers 36 and 31 connected from each end of I the resistor 30 toground, is provided to smooth the pulsating control potential'before itis supplied to the electrode 32 of the device 23. The time delay of animpulse transmitted through the filter comprising resistor 39 andcondensers 36 and 31 should be sufliciently small so that momentaryincreases of the amplified signal impressed through the' modulationtransformer i2 on the continuous potential from the source l3 may notoccur at such speed as to produce actual over-modulation. Since sometime delay is necessary in order to obtain smoothing action for thecontrol potential across the resistor 29, it is desirable that a controlpotential be produced before the unidirectional operating potentialapplied to the device II is reduced completely to only a portion of theunidirectional potential from the source l3. The condenser 21, which haslow impedance to audio signal currents, impresses substantially theentire amplified audio signal across the diode 28. These elements 25, 26and 21, which act to impress the operating potential on device 28 inreduced intensity and the signal voltage on device 28 in substantiallyfull intensity, are electrically passive. That is, they include nosource of electromotive force, and impress a bias potential on device 28derived entirely from the source It of operating potential. Theresistors 25 and 26 and the condenser 21 therefore impress theunidirectional potential component of the modulated anode potential forthe device ll upon the diode 29 in reduced intensity while impressingthe alternating potential component thereon at full intensity. The diode28 produces a control potential across the resistor 29 whenever thealternating potential component impressed on it exceeds in maximumintensity the continuous potential component impressed on it. A controlpotential is therefore developed across theresistor 29 whenever the peakintensity of the alternating potential applied to the anode ll of thedevice II exceeds a predetermined fraction of the unidirectionalpotential impressed there- It has been found desirable to adjust therelative magnitude of the resistors 25 and 26 so that a controlpotential is produced across the resistor 29 when the anode potentialfor the device II is reduced to about 10 per cent of the potential ofthe source l8. This degree of control v may conveniently be effected iithe resistor 26 be made about nine times as large as the resistor 25.The values of the resistors 25 and 26 may,

of course, be varied in any desired manner to zero. It is for thisreason that the modulation control means to effect this result is asfollows.

The resistors 25 and 26Iact together as a voltage dividing resistor toimpress across the diode 28 achieve control at other signal levels.

In Fig. 2 many of the circuit elements are similar to those illustratedin Fig. l and are given like reference characters. The anode modulatedtransmitter comprising the generator I4, the tuned circuit l5, thedevice II, and the tuned circuit l8 and coil I9 is not shown in detail.In this figure the diode 28, serially connected with the resistor 29, isconnected in shunt only to a portion of the resistor 26, rather than inshunt to the whole resistor 26 as is illustrated in Fig. 1. In the caseof a high power transmitter, where, for example, the source i3 may havea potential of about 2000 volts, a very large control potential may bedeveloped by the resistor 29 in the circuit of Fig.1. A very largecontrol potential is not necessary to control the variable gainamplifier 23.

In such a high power transmitter the circuit illustrated by Fig. 2 ispreferred, since the magnitude of the control potential developed acrossthe resisto 29 is very conveniently reduced to a suitable magnitude andthe diode 28 may accordingly be a very small, low voltage diode, as maybe used in the circuit of Fig. l in a low power transmitter. Byconnecting the diode 28 and the resistor 29 in shunt only to a portionof the resistor 26, there is impressed on diode 28 only a portion of theunidirectional potential across the resistor 26 and only a portion, inthe -fore appears at the same relative intensity of the unidirectionalpotential of the source l3 and the amplified audio signal.

In Fig. 3 many of the parts are identical with those illustrated inFigs. 1 and 2 and are given like reference characters. In the circuitillustrated by this figure, howeven the modulation transformer I2 isreplaced by a different sort of modulation transformer 38, whosesecondary winding is provided with an intermediate tap 39 and aplurality of tank 40 near one end. The other end of the secondarywinding of the transformer 38 is connected to the positive terminal ofthe source It, whose negative terminal is, as before, connected toground. The transmitter it] is connected to the intermediate tap 38 onthe secondary of the transformer 38 and the serially connected diode 28and resistor 25 are connected between ground and one of the taps 40 by aselector switch ll. As before, the diode 28 is connected with suchpolarity that the unidirectional potential from the source l3 tends tomaintain it in non-conductive condition.

The modulation control circuit of Fig. 3 operates somewhat difierentlyfrom those illustrated by Figs. 1 and 2. In this case the modulatedanode potential supplied to the transmitter I is altered beforeapplication to the diode 28 by increasing the alternating component oraudio signal thereof and maintaining the unidirectional componentconstant. As before, the diode 28 becomes conductive when thealternating potential component applied to it exceeds in maximumintensity the intensity of the unidirectional component applied thereto.Consequently, since the alternating potential component is increased inmagnitude before being applied to the diode 28, a control potential isdeveloped across the resistor 29 when the alternating potentialcomponent applied ,to the transmitter l0 reaches a maximum intensity,which is a predetermined fraction of the intensity oi. theunidirectional potential component applied to the transmitter It. Inother words, when the anode potential supplied through the transformer88 to the transmitter I0 is reduced by the amplified audio signal to apredetermined value greater than zero, a control potential is 45produced across the resistor 29.

In the circuit of Fig. 3 the secondary of the transformer 38 actuallyacts as an autotransformer to increase the intensity of the amplifiedaudio signal applied to the transmitter ll before impressing it on thediode :0. Since the secondary of the transformer 8| has a very smallresistance, the unidirectional potential of the source II is applied tothe diode 28 without substantial change in intensi By operation of theselector switch 4! the amount of increase in the amplified audio signal,which is applied to the diode 28, may be adjusted so that a controlpotential across the resistor 29 is produced at various levels of theamplified audio signal with respect to the unidirectional potential ofthe source l3.

The advantages of the modulation control circuit described above riseprincipally from the fact that the resistor 29 in any case is connecteddirectly to ground. The circuit arrangement which makes this possible,therefore, allows the use of a diode 28 of the low voltage type, suchfor example as a 6H6 diode or a 1V diode. It also allows the circuitsassociated with the amplifier device 23 to be connected to ground,

. rather than to a source of positive potential.

While I have shown a particular embodiment of my invention, it will, ofcourse, be understood that I do not wish to be limited thereto, sincediiferent modifications may be made both in the circuit arrangement andinstrumentalities employed, and I aim by the appended claim to cover anysuch modifications as fall within the true spirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

In a modulation responsive device for a high frequency amplifier havinga cathode electrode and another electrode between which unidirectlonaloperating voltage and modulating signal voltage are supplied, saidoperating voltage being subject to variations and the total voltagebetween said electrodes being subject to undesired reversal upon highpeaks of said signal voltage, a unilaterally conducting device, aninductance, means for supplying said operating voltage to said devicethrough said inductance with such polarity as to render said devicenonconductive so long as peaks of said signal voltage do not exceed saidoperating voltage, means for applying said signal voltage between saidelectrodes across a portion of said inductance, and means for supplyingsaid signal voltage across more of said inductance than said portion tosaid device, whereby said operating voltage and said signal voltage areapplied to said device with such relative intensities that said deviceis normally non-conductive but becomes conductive whenever said signalvoltage exceeds a predetermined constant fraction less than unity ofsaid operating voltage on said device.

GEORGE M. BROWN.

